Most power-generation systems produce heat as a by-product. For example, internal combustion engines used to power most vehicles today combust a high-energy fuel (e.g., gasoline) to generate mechanical motion and heat. Fuel cells that convert hydrogen and oxygen into electricity and heat are also being developed for a variety of applications, including power production for vehicles and electrical appliances. Other power-generation systems, such as bio-fuel processing, petroleum refining, industrial processing, and solar-thermal systems, to name a few, also produce heat as a by-product. At least some of the heat produced by such power-generation systems must be dissipated to the ambient environment.
Various cooling systems have been developed for dissipating heat. Automobiles, for example, may have as many as fourteen separate cooling systems, including cooling systems for the engine, oil, air conditioning system, and transmission. By way of illustration, most internal combustion engines are cooled by a liquid (e.g., water, antifreeze) that is circulated through a cooling loop provided in thermal contact with the engine. As the liquid is circulated, it absorbs heat generated by the fuel combustion. The cooling loop is connected to a heat-exchange system (e.g., a radiator). One type of automobile radiator may have a tube arranged in a parallel or serpentine manner among a series of copper or aluminum “fins” that are provided in thermal contact with the surrounding air. As liquid from the cooling loop flows through the tube, heat is conducted from the liquid into the air flowing past the fins (e.g., as the automobile moves).
The specific design and performance of currently available heat-exchange systems is dominated by the heat transfer characteristics of the materials from which these systems are made and convective heat transfer conditions on the fin surfaces. For example, typical automobile radiators may be fabricated from metals which have a relatively high thermal conductivity (e.g., aluminum, copper, etc.). However, these materials make the heat-exchange systems heavy, which negatively impacts the automobile's performance, fuel consumption, and emissions. Recent studies have shown that every twenty pounds-mass (lbm) of weight in current light-duty automobiles increases fuel use by 0.1 miles per gallon (mpg). In addition, typical heat-exchange systems have relatively high air intake or air loading requirements so that the liquid can be effectively cooled by the air flow. These loading requirements increase the surface area that must be exposed to the air flow, making the heat-exchange system large and cumbersome. Indeed, radiators are typically positioned at the front of the vehicle to maximize air flow to the radiator. Consequently, these loading requirements also increase drag on the automobile, negatively impacting the automobile's performance, fuel consumption, and emissions.
Other materials have also been studied for use with heat-exchange systems. For example, carbon foams and porous ceramics (e.g., silicon carbide) are highly conductive. Although these materials are light-weight and exhibit relatively high thermal-exchange properties, these materials are structurally weak. Therefore, widespread use of these materials in heat-exchange systems is unlikely, especially in heat-exchange systems used on-board automobiles.
Consequently, a need remains for a high performance heat-exchange system that is structurally sound and light-weight. Additional advantages would be realized if the surface area and/or frontal loading of the heat-exchange system were reduced. Fuel cell systems may also be improved if the heat-exchange system can be used to cool one or more components of the fuel cell directly.